One conventional manner of fabrication of a zener diode on an integrated circuit is to form the zener diode during the same processing steps used to form the bipolar transistors on the silicon wafer. Referring to FIGS. 1 and 2, the P+ anode region 11 of the diode is formed in the N epitaxial region 12 during the base diffusion of the bipolar transistor and the cathode region 13 is formed during the emitter diffusion, the diode structure also including the N+ buried layer region 14 and the isolation regions 15. At the time of diffusion of the cathode region 13, an N+ contact region 16 is diffused into the N epitaxial layer 12. A metallic interconnect 17 extending over the oxide layer 18 connects the region 13 with the epitaxial layer contact 16; a metal contact 19 is also provided for the anode region 11. The contact 17 is connected to the positive source of voltage supply and contact 19 is connected to the negative side.
As the voltage across terminals 17 and 19 increases, a point is reached where the zener diode avalanches and the current rapidly increases from zero to some maximum level. A typical voltage at which avalanche occurs is 6-7 volts.
One major problem with such forms of zener diode is that the voltage at which the avalanche occurs drifts with time. This is due to the fact that the junction avalanche initially starts at the point where the junction 20 abutts the oxide layer 18, i.e. at the surface of the semiconductor body. There is a fringing of the electric field at this point along the surface of any factor that influences this electric field will influence the zener voltage. The oxide will usually contain contaminants, for example sodium atoms; these sodium atoms have a plus charge and are very mobile in the oxide 18 even at room temperature. If these sodium atoms come near the junction 20, they bend the depletion layers at the surface and affect the breakdown voltage. Therefore, the breakdown voltage is a function of the electric field at the surface and may increase as a function of time, and the circuit may drift out of specification existing at first use.
A second disadvantage is that the zener voltage is dependent on the diffusion depths and other parameters of the diffused areas 11 and 13, and these in turn are a function of the deposition steps used to make the other transistors on the IC. Therefore the zener diode specifications are principally determined by considerations other than good zener diode parameters.